1. Field of the Invention
The present invention relates to the deposition of WSi.sub.x with a silane gas onto a semiconductor substrate. In particular, the present invention relates to the deposition of WSi.sub.x in order to lessen problems associated with peeling and cracking that may occur on the WSix layer after thermal processing of the semiconductor substrate.
2. Description of the Related Art
An epitaxial growth process is used in semiconductor wafer fabrication in order to deposit a thin layer (e.g., 0.5 to 20 .mu.m) of a single crystal material upon a surface of a single crystal substrate, such as silicon.
Epitaxial growth can be accomplished from a vapor phase (VPE), a liquid phase (LPE), or from a solid phase (SPE). For silicon substrates, VPE is the most widely accepted method for achieving epitaxial growth.
One important motivation for developing epitaxial growth processes is to improve the performance of bipolar and field effect transistors, as well as other types of integrated circuits. By growing a lightly doped epitaxial layer over a heavily doped silicon substrate, the bipolar device can be optimized for high breakdown voltage of the collector-substrate junction, while still maintaining low collector currents.
One type of epitaxial deposition process is known as chemical vapor deposition (CVD). As described in "Silicon Processing for the VLSI Era", Volumes I-III, by Stanley Wolf (1986 edition), the following five steps are fundamental to all CVD processes: 1) the reactants are transported to the substrate surface, 2) the reactants are adsorbed on the substrate surface, 3) a chemical reaction takes place on the surface leading to the formation of the film (the "product") and reaction by-products, 4) the reaction by-products are desorbed from the surface, and 5) the by-products are transported away from the surface.
S. Wolf's book also states that there are four major chemical sources of silicon used commercially for epitaxial deposition: 1) silicon tetracholoride (SiCl.sub.4), 2) trichlorosilane (SiHCl.sub.3), 3) dicholorosilane (SiH.sub.2 Cl.sub.2), and 4) silane (SiH.sub.4). Of these chemical sources, silane has been the most widely used.
Defects may occur due to the epitaxial deposition, such as dislocations and stacking faults. Dislocations are generated in epitaxial films by several mechanisms, including: a) the propagation into the growing film of a dislocation line in the substrate that reaches the substrate surface, b) the existence of a large difference in lattice parameter between the film and the substrate, and c) by thermally generated stresses that exceed the yield strength of the silicon, resulting in a slip. Stacking faults result from two distinct causes: a) microscopic surface steps on the surface, and b) impurities either on the surface or within the reactor itself. See S. Wolf, Volume I, pages 140-141.
Defects can be reduced by using radiant heaters or by using low lamp power, which, eliminate the electrostatic migration of particles. Also, defects in the epitaxial layers can be reduced by the use of denuded zones and oxygen intrinsic gettering of the substrates prior to deposition. Still another way of reducing defects is to use strain layers. Each of these techniques is discussed in detail in the S. Wolf reference listed above.
The use of SiCl.sub.4 requires high deposition temperatures (currently in a range of from 1000-1200.degree. C.), which results in significant autodoping and outdiffusion. An advantage of using SiCl.sub.4 is that very little deposition occurs on the reactor walls, thereby reducing the frequency of cleaning. Thus, SiCl4 is useful for thick deposits (&gt;3 .mu.m) on devices that can tolerate the elevated temperature.
SiHCl.sub.3 has similar properties to SiCl.sub.4, and it is also deposited at high deposition temperatures. SiHCl.sub.3 is used primarily for depositing thicker epitaxial layers.
SiH.sub.2 Cl.sub.2 is used to deposit thinner layers at lower temperatures (currently in a range of from 500-600.degree. C.). The lower temperature reduces outdiffusion and autodoping. Typical, films prepared from SiH.sub.2 Cl.sub.2 have lower defect densities than those prepared either from SiH.sub.4 or SiCl.sub.4.
SiH.sub.4 is used to deposit very thin epitaxial layers at lower temperatures (currently in a range of from 350-475.degree. C.). This results in substantially less autodoping and outdiffusion when compared to films deposited at higher temperatures using other chemical sources. A disadvantage of SiH.sub.4 is that frequent cleanings of reactor walls are typically required, since it can pyrolize (decompose) at low temperatures, causing heavy deposits on reactor walls.
In order to effectively utilize refractory metal silicides in VLSI fabrication, stringent metal requirements must be satisfied. Refractory metals that meet these requirements include Titanium (Ti), Tantalum (Ta), Molybdenum (Mo), Tungsten (W), and Platinum (Pt). In general, silicides can be formed by three techniques, a) deposition of the pure metal on silicon (i.e., onto single crystal and/or polycrystalline Si), b) simultaneous evaporation of the silicon and the refractory metal from two sources (co-evaporation), and c) sputter-depositing the silicide. The advantages and disadvantages of each of these techniques are detailed by S. Wolf, Vol. I, pages 388-392.
In the sputtering method, composite targets for silicide deposition are generally manufactured by power metallurgical technique, which employ a mixture of particles of metal (e.g., tungsten) and Si. The powders are pressed together, and sintered at high temperatures and pressures, to 70-80% of their theoretical density.
One way to produce a WSi.sub.2 layer onto a semiconductor substrate is to perform a CVD process with WF.sub.6 vapor and silane vapor. The following chemical process results: EQU WF.sub.6 (vapor)+2SiH.sub.4 (vapor).fwdarw.WSi.sub.2 (solid)+6HF+H.sub.2
Other systems for CVD deposition of WSi.sub.2 have been developed, including single-wafer, plasma-enhanced deposition machines. These other systems are described in detail in Volume I, Chapter 6 of the S. Wolf reference listed above.
Extensive efforts have been directed towards developing a CVD of tungsten (W) thin films for refractory metal interconnects for VLSI. CVD tungsten is a good candidate for interconnect applications, because of its low resistance, low stress, excellent conformal step coverage, and because its thermal expansion coefficient closely matches that of silicon. Some disadvantages of W include: a) poor adhesion to oxides and nitrides, b) oxides form on W at temperatures &gt;400.degree., and c) silicides form at temperatures &gt;600.degree. C. Details of the advantages and disadvantages are given in Volume I, pages 399-400 of the S. Wolf reference.
In VLSI fabrication, a particular problem has arisen in the formation of high density conductive lines, due to the prevalence of cracking and gaps forming in those lines. This problem is particularly evident in integrated circuit memory arrays where frequent cracks have been found in word lines, thereby directly affecting the reliability of the integrated circuit memory.